what makes their biology so different from aquatic arthropods? no problems with size there.

Aquatic animals of all kinds get a boost from the buoyant force in water and in general can grow to be much larger than land mammals (think of whales or countless other examples). The land mammals have a limit in that the cross area of their feet or arms used to support their weight when standing only grow by the square of the length measurement while their volume increases by the cube of the length measurement. Therefore, size grows at a faster rate than the limbs designed to support it. Most of the large land animals that you do see (like Elephants) have to have extremely thick legs in order to support such a large mass on land. On the other hand, extremely large sea animals can simply displace large amounts of water in order to prop themselves up. You also see larger animals in sea water (which contains lots of salt) as opposed to fresh water. The main reason is that sea water has a larger density than fresh water, and it gives a larger buoyant force when you displace an equivalent amount of water.

I remember reading somewhere that the weight of the exoskeleton was a major limiting factor to the size of the arthropods. Apparently the presence of the exoskeleton magnifies the whole cubed vs. squared issue of size growth as the exoskeleton would grown significantly in "relative weight" if the size of the arthropod increased.

The limits on the size of most land athropods is that they breathe by diffusion of the air through spiracles on the sides of the body. Without lungs to drive the air flow it can only diffuse a very small distance.
Spiders (some?) have a slightly more efficent system of flat surfaces - book lungs.

I'm not entirely sure that a lower oxygen supply would have to limit the size in and of itself. There are rather large animals that live entirely underwater for example and the oxygen supply down there is much much lower than it is anywhere on land (most of them employ specialized organs that help them cope with this). If you go by evolutionary theory, you can ask why didn't the arthropods evolve similar tricks to help them cope with the oxygen levels on land?

What is interesting in general is the modern reversal from the trend in the "super sized" animals that lived in our distant past such as the dinosaurs. The lizard and bird like dinosaurs all have taken on smaller shapes in modern times when compared to the fossil record of their dino-analogs. Sorta reminds me of how the computers in the 70's were these big behemoths only to be replaced by dinky little hard drives that can fit in the palm of your hand.

Structures like gills are easier to make underwater where you don't have to worry about supporting them. Evolution is funny - often the best solution is impossible because of the 'you can't get there from here' problems of unevolving existing body parts.

An interesting aspect of the dinosaurs is to consider something like a pack of lions/wolves as the equivalent of a large carniverous dinosaur - it turns out that the mass of carnivore in an area is about the same, it's just spread out in more bodies.

An interesting aspect of the dinosaurs is to consider something like a pack of lions/wolves as the equivalent of a large carniverous dinosaur - it turns out that the mass of carnivore in an area is about the same, it's just spread out in more bodies.

It is an interesting point and if you think about it it seems like the pack of lions has a distinct advantage by choosing to be a bunch of little carnivores instead of one huge dinosaur carnivore. Let's say a single carnivore dies in the pack of lions. The pack as a whole in that area is largely unaffected by this. On the other hand, if a single dinosaur dies it is the equivalent of a carnivore holocaust in that area. Perhaps it is in this way that the smaller animals eventually won out over the larger animal types. After the meteor hit and wiped out the dinosaurs even if it also wiped out a good 90% of the small animals, there would still be plenty left to rebuild the population in a "Dr. Stangelove" sort of way...